The presented invention relates generally to optical disc-like recording medium and more particularly to such an optical disc-like recording medium mixedly having thereon a read-only region, or a data-prerecorded region, (which will be referred hereinafter to as a ROM region) and a write-allowable region (which will be referred hereinafter to as a RAM region) and a method of manufacturing the optical disc-like recording medium.
In order to meet the recent requirement for recording information on a recording medium with a high density and reproducing the information therefrom, optical disc-like recording media such as an optical compact disk are being developed which are arranged such that an information signal is recorded by illuminating a light beam intensity-modulated in accordance with the information signal. For reproducing the information signal from the recording medium, an accurate trace of the information-recorded track is made under a tracking control technique, such as for example by a tracking error detection system using the so-called push-pull method. The compact disk comprises a train of pits with a depth determined in a specific relation to light whose wavelength is 780 nm, the pit train corresponding to the recording information signal. The signal surface of the compact disk is covered by a thin film made of an aluminum so that the reflection factor of the land portion of the compact disk becomes 70 to 90%. The reading of the information signal from the compact disk is made by illuminating a light spot whose wavelength is 780 nm. The reading of the information signal from the compact disk is effected by utilizing the fact that the intensity of the reflection light from the pit portion on the signal surface is less than the intensity of the reflection light from the land portion because of the interference of light occurring at the pit portion. For tracking control the tracking error information is detected from the asymmetry occurred in a reflected light spot when the light spot for reading trails off the pit train.
Like the widely used compact disks, an optical disc-like recording medium (which will be referred hereinafter as an optical disk) has been developed which is reproducible through a reproducing apparatus of compact disks. One example of an optical disk being interchangeable with a compact disk produced by a compact disk reproducing apparatus is the direct read after write optical disk (which will be referred hereinafter to as a write type optical disk) that has a reproduction-only region (ROM region) and a write-allowable region (RAM region), or only the RAM region. The interchangeability of the optical disk with the compact disk is required in order to satisfy predetermine reproducing conditions such as the reflection factor, modulation factor and tracking signal output in connection with the compact disk.
A description is given hereinbelow of the problems which arise when arranging the optical disk to satisfy the predetermined reproducing conditions. The standardized value of the reflection factor of the compact disk is required to be above 70% when viewed from the reading side in the case where a laser beam of 780 nm is radiated from the reading side. Since the reflection loss of about 8% occurs on the disk surface, the reflection factor on the metallic reflection film of the disk is required to be at least above 80% in order to keep the reflection factor at the reading side to above 70%. Generally, the aluminum reflection film of the compact disk is arranged so as to offer a reflection factor of above 80% which satisfies the condition on the reflection factor. However, for the write type optical disk, absorption of optical energy into the recording film occurs in recording. In addition, there is a guide groove for the tracking control in recording provided in the transparent substrate. Thus, there is also a loss of the light intensity due to the incident light being diffracted by the guide groove. Therefore, it is difficult to adjust the reflection factor at the reading side of the optical disk to the standardized value of the compact disk.
Secondly, to arrange the write type optical disk to be interchangeable with the compact disk which can satisfy the condition on the modulation factor of the high-frequency signal, there is the following problem. Although a compact disk can easily satisfy the standardized value of the modulation factor of a high-frequency signal because it is arranged to read the information signal by using diffraction of light to the pits thereon, the information recorded onto an optical disk is effected with hole-formation, phase-variation and other methods which allow the information signal to be read on the basis of variation of the reflection factor. Therefore, it is difficult for the conventional optical disk to satisfy the standardized value of the modulation factor of the high-frequency signal for a compact disk because the difference between the reflection factor of the land portion, i.e., the non-recorded portion, and the recorded portion (corresponding to the pit), formed due to the hole-formation or the phase-variation, is small. This is because the modulation factor of the high-frequency signal is small. In order to satisfy the standardized value of the modulation factor of the high-frequency signal for the compact disk, the optical disk is also required to have an arrangement to perform the reading of the information signal on the basis of the phase structure such that the reading of the information signal is effected with light diffraction due to the pit.
Furthermore, to arrange the optical disk to be interchangeable with the compact disk to satisfy the standardized value in terms of the output level of the tracking signal, the following problem arises. In the optical disk the output level of the tracking signal generally depends upon the phase structure determined in accordance with the configuration of the guide groove for the tracking control. In the case that the recording of information to the recording film is made by hole-formation, the hole causes turbulence of the phase structure, thereby making it difficult to obtain the tracking signal with a desired output level.